Several reviews of the toxicology of mercury have appeared recently (1-4).
In this present review I do not repeat all the material presented in
these extensive reviews. Instead I focus on three chemical species of
mercury that are currently the source of intense public health
interest.

Public health concerns about methyl mercury in
edible tissue of fish suddenly erupted in 1969 when fish from Lake St.
Clair bordering Michigan were found to have high levels. This and
other findings discussed in this review have maintained public health
concerns over this form of mercury. In 1997, the U.S. Environmental
Protection Agency (U.S. EPA) reduced recommended safe intakes of
methyl mercury by a factor of about five (1), which brought public apprehension to new heights.

The
U.S. EPA-recommended safe
intake level is referred to as the reference dose and is defined as
that dose that can be absorbed daily for a lifetime without a
significant risk of adverse effects. The new reference dose was
estimated in 1997 to be 0.1 µg methyl mercury/kg body weight/day. This
dose implies that the amount of methyl mercury ingested in just one
7-oz can of tuna fish per week would equal or even slightly exceed the
new limit, depending on the consumer's body weight. Other federal
regulatory agency guidelines allow higher levels (4): The U.S.
Food and Drug Administration (FDA) guideline is equivalent to 0.5 µg
Hg/kg/day, and that of the Agency for Toxic Substances and Disease
Registry (ATSDR) is 0.3 µg Hg/kg/day.

Mercury amalgam tooth
fillings have been used since the early nineteenth century.
Periodically, debates have arisen about the potential danger from
mercury. These debates are sometimes referred to as
the "amalgam wars." The most recent began with an observation in the
1970s that mercury vapor was released from amalgam, especially during
the process of chewing, and that this vapor could be inhaled.
Concentrations of mercury vapor measured in the air of the oral cavity
approached and even exceeded occupational health limits. The debate
has now reached new heights (or lows, depending on the side of the
argument) with claims that chronic degenerative diseases of the
nervous system such as Parkinson's disease and Alzheimer's disease are
caused or exacerbated by mercury released from amalgam.

In
the late summer of 1999, concern was expressed by the American Academy
of Pediatrics and by the U.S. Public Health Service about the safety
of a mercury preservative used in many vaccine preparations routinely
administered to infants (5). Within about 18 months, the mercury preservative was removed by the
manufacturers from all vaccines destined for use in the United States.

The mercury preservative has the molecular formula CH3CH2-Hg-S-C6H4-COOH. This preservative was introduced into vaccines in the early 1930s and has been used ever since (6). It was given a clean bill of health by the FDA in 1976 (7).
However, the U.S. EPA later lowered its allowable safe long-term
daily intake for mercury, as discussed above. As a result, a more
recent review of thimerosal by the FDA raised questions about possible
health risks.

My objective in this review, therefore, is to
give the toxicologic background for these three species of mercury
and the public health issues surrounding them. In each case, I address
human exposure, disposition in the body, and adverse effects. Where
possible, I discuss the underlying mechanisms. Emphasis is
on the human target. I also discuss ecologic aspects only inasmuch as
they may play a role in human exposure.

Quantitative
estimates of human health risks are not made in this review. Such a
task is left to "expert committees" covering a range of disciplines
that cannot be mastered by one individual. Nevertheless, the
toxicologic background presented here should give at least a qualitative
idea of the type of health risks we face from these forms of
mercurys.

The first methyl mercury compounds were synthesized in a chemical laboratory in London in the 1860s (8).
Two of the laboratory technicians died of methyl mercury poisoning.
This so shocked the
chemical community that methyl mercury compounds were given a wide
berth for the rest of the century. However, early in the twentieth
century the potent antifungal properties of the short-chain alkyl
mercury compounds were discovered, leading to application to seed
grains, especially for cereal crops. The widespread global use of
these mercury compounds was found to be highly protective of seed
grain from what otherwise would be devastating fungal infections and the
loss of the grain harvest.

Despite this widespread use,
few cases of poisoning were reported for the first half of the
twentieth century. However, in the late 1950s and early 1960s serious
outbreaks of alkyl mercury poisoning erupted in several developing
countries (9). The largest, most recent outbreak occurred in rural Iraq in the winter of 1971-1972 (10). Some 6,000 cases were admitted to hospitals. An epidemiologic
follow-up suggested that as many as 40,000 individuals may have been poisoned.

These
outbreaks were caused by preparing homemade bread directly from the
treated seed grain. Several factors contributed to these mass health
disasters. The warning labels were not written in the local language.
Well-known symbols for poisons in the Western world, such as the skull
and crossbones, have no meaning to rural Arabs unfamiliar with stories
of "pirates on the Spanish Main." Typically, a red dye is added to the
treated grain to indicate the presence of a fungicide. This was
counterproductive, as the victims washed away the dye, thinking they
had also removed the poison. The insidious properties of methyl
mercury were another important factor, as there is a long latent
period between ingestion and the first appearance of symptoms (10).

Also in the late 1950s, evidence emerged of
environmental damage from treated grain (11). It was observed
in Sweden that predatory birds were developing neurologic disorders.
These birds were at the top of a food chain that began with small
mammals consuming the treated grain freshly planted in the fields.
Analysis of feathers from museum-preserved birds indicated a sharp
rise in mercury levels at the time when mercurial compounds were
introduced as agricultural fungicides. Because some of these birds
were migratory, it was possible to show that elevated mercury levels
were found only in those feathers that grew when the birds were in
Sweden.

As a control measure, the Swedish investigators
decided to check mercury levels in the feathers of fish-eating birds,
where mercury levels were assumed to be low. To their astonishment,
mercury levels were elevated despite that these birds had no dietary
connection with the treated grain. Eventually
this finding led to a landmark discovery on the environmental fate of
mercury, namely that microorganisms in the aquatic environment are
capable of converting inorganic mercury to methyl mercury. This is the
first step in the aquatic food chains, where methyl mercury
bioaccumulates in higher organisms to plankton, herbivorous, and finally
in the top fish predators such as sharks and fish-eating marine
mammals. A similar food chain exists in bodies of freshwater, with
such species as pike and bass having some of the highest levels of
methyl mercury.

The potential for bioaccumulation in aquatic
food chains was demonstrated dramatically in two outbreaks of human
poisoning in Japan at about this time. The Japanese health authorities
in Minamata had been aware for some time that fishermen and their
families were suffering from a neurologic disease, exhibiting signs of
incoordination, constricted visual
fields, and numbness in the extremities. The cause was elusive until a
visiting physician from Scotland recognized the neurologic signs and
symptoms from cases of occupational methyl mercury poisoning he had
seen in England in 1939 (8). Eventually the source in Japan was
traced to a factory manufacturing acetaldehyde, where inorganic
compounds of mercury were used as a catalyst. The producers were
unaware that the synthetic process converted some of the mercury to
methyl mercury, which was discharged into Minamata Bay. It was
difficult to believe that methyl mercury released into a large ocean
bay could be bioaccumulated to such an extent that the fish carried
levels of methyl mercury that would prove lethal when consumed by
humans.

Global Cycling of Mercury

The twin
discoveries of biomethylation and bioaccumulation aroused intense
interest in the environmental fate
of mercury and in pathways to human exposure. Methyl mercury was soon
detected in all species of fish and in fish-consuming animals. The
source appeared to be inorganic mercury biomethylated by
microorganisms in sediments of both fresh and ocean water.

Many
anthropogenic sources were identified. Chloralkali plants discharged
inorganic mercury as waste into rivers, lakes, and ocean bays. Paper
pulp factories likewise discharged a variety of mercury compounds used
a slimicides. These practices now have been eliminated, but
contamination of aquatic sediments now occurs worldwide because of
extensive goldmining operations, for example, in the Amazon basin (12).

Large
quantities of liquid mercury are used to extract the sedimentary gold
found in river beds. Pure gold is recovered when the mercury is
evaporated from the amalgam by heating. It has been estimated that over
130
tons of mercury have been released each year into the Amazon basin
alone (13).

The global cycling of mercury begins
with the evaporation of mercury vapor from land and sea surfaces.
Volcanoes can be an important natural source (14). The burning
of fossil fuel, especially coal and municipal waste incineration, is a
major anthropogenic source to the atmosphere. Mercury vapor is a
chemically stable monatomic gas. Its residence time in the general
atmosphere is estimated to be about 1 year. Thus, mercury vapor is
globally distributed even from point sources. By processes not yet
fully understood, the vapor is oxidized in the upper atmosphere to a
water-soluble ionic mercury, which is returned to the earth's surface in
rainwater. This global cycling of mercury results in the distribution
of mercury to the most remote regions of the planet. For example,
environmental mercury levels even in the
arctic water may not differ greatly from levels in more southern
latitudes.

The global cycling of mercury, along with the
processes of biomethylation and bioaccumulation, implies that humans
must have consumed methyl mercury in fish dating back to times before Homo sapiens evolved (15).
It could be argued that environmental levels of mercury vapor were
much higher in an earlier period of the earth's history when oxygen had
not yet appeared in the atmosphere. As levels of oxygen began to rise,
increasing amounts of the vapor would be converted to the ionic form.
Life forms at those Archean times had to protect themselves not only
from this new toxic gas, oxygen, but also from ionic mercury pouring
down in rainwater. Perhaps it is no coincidence that those proteins
and antioxidant molecules present in today's cellular machinery to
protect against oxygen also are our main line of defense
against mercury.

Disposition in the Body

The U.S. EPA (1,2) and ATSDR (3)
in recent reviews give extensive details on the disposition of methyl
mercury in the body. A brief review and update are provided here.

About
95% of methyl mercury ingested in fish is absorbed in the
gastrointestinal tract, although the exact site of absorption is not
known. It is distributed to all tissues in a process completed in about
30 hr. About 5% is found in the blood compartment and about 10% in
brain. The concentration in red blood cells is about 20 times the
concentration in plasma. Methyl mercury crosses the placental barrier.
Levels in cord blood are proportional to but slightly higher than
levels in maternal blood. Levels in the fetal brain are about 5-7
times that in maternal blood (16). Brain-to-blood ratios in adult humans and other
primates are approximately in the same range.

Methyl
mercury avidly accumulates in growing scalp hair. Concentrations in
hair are proportional to simultaneous concentrations in blood but are
about 250 times higher. They are also proportional to concentrations
in the target tissue, the brain (16). Longitudinal analysis of strands of scalp hair can recapitulate past blood and brain levels (17).
Hair and blood are used as biologic indicator media for methyl
mercury in both the adult and fetal brain (in the latter case, maternal
hair or cord blood).

Methyl mercury is slowly metabolized
to inorganic mercury mainly by microflora in the intestines, probably
at a rate of about 1% of the body burden per day. Some demethylation
also occurs in phagocytic cells The biochemical mechanism is unknown.
Although methyl mercury is the predominant form of mercury during
exposure,
inorganic mercury slowly accumulates and resides for long periods in
the central nervous system. It is believed to be in an inert form,
probably insoluble mercury selenide (18).

Urinary
excretion is negligible, of the order of 10% or less of total
elimination from the body. Methyl mercury undergoes extensive
enterohepatic cycling. It is secreted into bile and partly reabsorbed
into the portal circulation and thereby returned to the liver. A
fraction of the biliary mercury is converted by microflora to
inorganic mercury. The latter is reabsorbed only to a small extent.
Thus, most of the methyl mercury is eliminated from the body by
demethylation and excretion of the inorganic form in the feces. The
processes of biliary secretion and demethylation by microflora do not
occur in suckling animals. The role of these two processes in suckling
human infants is unknown.

The high
mobility of methyl mercury in the body is not due to lipid
solubility, as claimed in some textbooks. Methyl mercury is present in
the body as water-soluble complexes mainly if not exclusively attached
to the sulfur atom of thiol ligands. It enters the endothelial cells
of the blood-brain barrier as a complex with l-cysteine. The process
is so specific that the complex with the optical isomer d-cysteine is
not transported. Structurally, the l-complex is similar to the large
neutral amino acid l-methionine and is carried across the cell
membrane on the large neutral amino acid carrier (19).

Methyl
mercury is pumped out of mammalian cells as a complex with reduced
glutathione. For example, it is secreted into bile as a glutathione
complex. The glutathione moiety is degraded in the bile duct and gall
bladder to a dipeptide and finally to the l-cysteine complex.
Presumably, in this form it is
reabsorbed into the bloodstream to be returned to the liver, thereby
completing the enterohepatic cycle (20-22).

The
elimination of methyl mercury from the body approximates first-order
kinetics. Half-times vary from one tissue to another but generally
fall in the range of 45-70 days. Thus, individuals with long-term
regular exposure to methyl mercury attain a steady-state body burden
in about 1 year (five half-times).

Several thiol-containing
complexing agents have been successfully used to remove methyl mercury
from the body [e.g., in the Iraq outbreak; see Clarkson et al. (23)].
An interesting example is a thiol-containing resin that, when given
by mouth, traps the methyl mercury secreted in bile and carries it
into the feces. Perhaps the most promising complexing agent is N-acetylcysteine (24). It enhances methyl mercury excretion when given orally,
has a low toxicity, and is widely available in the clinical setting.

Adverse Effects

The
major toxic effects of methyl mercury are on the central nervous
system. Its toxic action on the developing brain differs in both
mechanism and outcome from its action on the mature organ, so the two
actions are treated separately here [for detailed reviews, see U.S. EPA
and ATSDR (2,3)]. However, recent reports have raised the
possibility that methyl mercury may have adverse effects on other
targets in the body.

The mature central nervous system. The
action of methyl mercury on adults is characterized by a latent
period between exposure and onset of symptoms. The period can be
several weeks or even months, depending of the dose and exposure
period. Perhaps the most dramatic example of latency was in the case
of severe, ultimately fatal
poisoning of a chemistry professor from exposure to dimethyl mercury (25).
A single exposure from a spill of liquid dimethyl mercury took place
in August. The professor continued her normal professional work
without any apparent ill effects. In November she presented a paper at
an overseas conference. It was not until late December that the first
symptoms appeared. Within a few weeks the full syndrome of severe
methyl mercury poisoning became manifest. Despite many decades of
research on methyl mercury toxicology, the mechanism underlying this
long latent period is still unknown.

Paresthesia, a numbness or a "pins and needles" sensation, is the first symptom to appear at the lowest dose (10).
This may progress to cerebellar ataxia, dysarthria, constriction of
the visual fields, and loss of hearing. These signs and symptoms are
caused by the loss of neuronal cells in specific anatomical
regions of the brain. For example, ataxia results from the loss of
the granule cells in the cerebellum. The neighboring Purkinje cells
are relatively unaffected.

The mechanism underlying the focal damage to the adult brain is still not established with any certainty. Syversen (26)
examined the effect on protein synthesis in various areas of brain of
rats poisoned with methyl mercury. Protein synthesis was inhibited in
all three areas studied--the granule and Purkinje cells of the
cerebellum, and the cells from the cortical areas of the brain.
Protein synthesis recovered in two types of neurons but not in the
granule cells. These data suggest that the focal damage to the brain
is not due to the initial insult but depends on the capacity of
neuronal cells for repair, as suggested by Jacobs et al. (27). Apparently the small granule cells lack the repair systems present in the other larger
cells. Sarafian et al. (28) have suggested that the selective
vulnerability of cells in the nervous system may arise from a
"critical absence of inherent protective mechanisms."

Cellular
defenses may be decisive in determining the toxic outcome and deserve
further study. If we understand the defense mechanism, we may be able
to predict which individuals are most susceptible. Thiol compounds
probably play a key role (29). Resistant cells have higher levels of the thiol-containing peptide glutathione (30). Glutathione also plays a key role in the excretion of methyl mercury [for further discussion, see Sarafian et al. (28)].

Selenium is a dietary component that may affect the disposition and toxicity of methyl mercury. Ganther et al. (31)
were the first to observe that selenium compounds could delay the
onset of toxic effects in animals fed
methyl mercury in tuna. This gave rise to a series of studies by his
group and others. However, despite promising indications from animal
studies, no definite studies have yet been carried out on human
populations co-exposed to methyl mercury and selenium [for a recent
review, see National Research Council (4)].

Methyl
mercury is converted to inorganic mercury in the brain. It is possible
that the inorganic ion is the proximate toxic agent responsible for
the brain damage. However, experiments on rats comparing methyl and
ethyl mercury compounds suggest that the intact methyl mercury radical
is the toxic agent (32). Ethyl mercury converts to inorganic
mercury more rapidly than methyl mercury, but the latter produces more
severe brain damage.

Autopsy samples taken years after exposure to methyl mercury reveal that inorganic species account for most if not all of the
remaining mercury in the brain (33). It has been suggested
that the long residence time is due to inorganic mercury forming an
insoluble complex with selenium (18). However, Charleston et al. (34)
have challenged this view, suggesting that inorganic mercury released
in brain tissue from methyl mercury may be the proximate toxic agent.
The toxicologic role of inorganic mercury remains a matter of debate.

Other adverse effects in adults. Most epidemiologic studies and clinical reports on adults [for review, see WHO (18)]
have identified neurologic signs and symptoms of poisoning associated
mainly with the central nervous system. An important exception is an
extensive study on the relationship between fish consumption, levels
of mercury in urine and scalp hair, and risk of cardiovascular disease
in adult male residents living in eastern Finland (35). A
statistically significant correlation was found between mercury
levels and cardiovascular disease even after correction for numerous
cardiovascular risk factors. A subsequent study by the same group
found a correlation between mercury accumulation and accelerated
progression of carotid atherosclerosis (36).

However, it is difficult to draw firm conclusions. Stress, believed to be a major risk factor (37),
was not directly measured. The highest recorded hair level of 15.7
ppm was more than six standard deviations from the mean. A histogram
of hair levels was not presented, but these statistics imply that a
small percentage of the study group had high mercury levels. Outlying
and "influential points" may play a major role in studies of this type
[e.g., Myers et al. (38)]. It would have been of interest to see if these correlations persisted when the very high mercury levels were
excluded.

Given the serious health implications, a repeat of
this study in another population is needed. If these findings are
confirmed, two long-held dogmas may have to be abandoned, namely, that
methyl mercury primarily affects the central nervous system and that
the prenatal period (see below) is the most susceptible part of the
life cycle.

Effects on the developing brain. The
first indication of the special susceptibility of the developing
brain to prenatal exposure to methyl mercury came from anecdotal
reports from Minamata that mothers with mild symptoms gave birth to
offspring with severe brain damage. The Iraq outbreak confirmed that
severe brain damage can occur from high prenatal exposure. A milder
syndrome was also identified in the Iraq outbreak (39).
Children apparently normal nevertheless had a history of delayed
achievement of developmental
milestones and, on examination, exhibited neurologic abnormalities
such as brisk tendon reflexes. When the prenatal exposure was
determined from mercury levels in maternal hair samples, it was
possible to construct a dose-response relationship between peak hair
mercury levels in pregnancy versus number of abnormal offspring
showing developmental delays and abnormal neurologic findings (39,40).

This
study was of interest for two reasons. First, a dose-response
relationship has been established for prenatal exposures to a
toxicant; that is, a dose to the mother predicts the probability of
effects in her offspring. This discovery laid the groundwork for
further quantitative estimate of prenatal risks from methyl mercury.
No doubt this relationship was made possible by the parallel between
levels of mercury in maternal and fetal tissues. Indeed, it was later
demonstrated in another study that maternal
hair levels of mercury were proportional to levels in autopsy samples
of brain tissue from infants who died shortly after birth (16). Ernhart et al. (41),
at virtually the same time, published a dose-response curve for
prenatal exposures to ethanol. Probably as with methyl mercury, the
high mobility of ethanol ensures that maternal levels predict those in
the fetus.

A second unique aspect of the Iraqi study (40)
was the application of continuous single-strand hair analysis to
determine peak levels during pregnancy. By the use of X-ray
fluorescence analysis, it was possible to measure the concentration of
mercury in contiguous 2-mm segments of a single strand of maternal
hair, thus giving a complete picture of mercury levels in pregnancy.
Moreover, because exposure in Iraq took place over a single period of
time, it was possible to fit the hair data with a single compartment
model
covering both the rising levels during intake and the exponential
fall afterward. This allowed the true peak value to be calculated from
the curve by fitting all the data points, as opposed to taking the
single highest value, which would be more prone to error. It is
unfortunate that this method of analysis was not used in subsequent
studies of prenatal exposure.

The studies of the Iraq
outbreak confirmed what had been suspected from the outbreak in Japan,
that the fetal brain was more sensitive than the mature organ. A
Swedish expert group (11) had estimated a threshold level for
neurologic effects in adults at about 50 ppm in hair, an estimate
confirmed by the findings in Iraq (10). This level may be
compared with an estimated threshold as low as 10 ppm for prenatal
effects (milestones of development and neurologic change) in Iraq (40).
As these studies were being conducted and
early findings presented at scientific meetings, concern arose that
methyl mercury in fish normally consumed in our diet might present
risks of prenatal damage. Several large epidemiologic studies were
conducted in people consuming freshwater fish (42) and ocean water fish (43), and large-scale studies are continuing even to this day focusing on neuropsychologic development [e.g., (44,45)].
These studies have not yet provided a consistent picture of the
lowest prenatal levels that offer a measurable risk of damage to the
developing brain. However, at this time it can be said that these
studies on fish-eating populations taken as a whole are consistent
with the original findings in Iraq that effects can be detected in the
range of 10 ppm in maternal hair. Indeed, a U.S. EPA reference dose
published recently (2) is identical to the previous estimate from the Iraq data (1).

Mechanism of prenatal damage. Several
studies have given some insight into the mechanism underlying
prenatal brain damage. Autopsy brain samples from the Minamata
outbreak indicated widespread damage to all areas of the fetal brain,
as opposed to the focal lesions seen in adult tissue. Microcephaly was
also observed (18). Autopsy tissue from Iraq also gave invaluable clues to the nature of prenatal brain damage (46).
The normally ordered parallel arrays of neuronal cells in the cortex
were found to be disrupted, which is indicative of a general
disturbance in the developmental growth of the brain. Moreover,
neurons were present such as Purkinje cells that had failed to migrate
to the cerebellum. These findings from both Japan and Iraq indicated
that the most basic processes in brain development were affected,
namely, neuronal cell division and migration.

Experimental work in animals and in vitro has provided a mechanism explaining why methyl mercury inhibits both cell division and migration (47-49).
These studies show that the cytoarchitecture first affected at the
lowest levels of methyl mercury is the microtubular system. Intact
microtubules are required for both cell division and migration.
Microtubules are formed by a treadmilling process whereby assembly from
-and
ß-tubulin monomers occurs at one end and disassembly at the other.
Apparently, methyl mercury binds to thiol ligands (-SH) groups on the
tubulin monomers and blocks the assembly process. The disassembly
continues unchanged, thus leading to the complete loss of the tubule.

Other adverse effects of prenatal exposures.
Studies in 7-year-old children
revealed an elevation in both systolic and diastolic blood pressure
that correlated with prenatal exposure to methyl mercury (50).
The study was conducted in the Faroe Islands on a large cohort of
children whose mothers had ingested methyl mercury mainly from whale
meat but also from fish. This effect is seen only at the lower range of
blood levels from about 1 to 10 µg Hg/L. Above this range no further
increase is seen, even at blood levels in the mother ranging as high
as 250 µg Hg/L.

As elevated blood pressure in children may be
indicative of later cardiovascular problems, this finding is of
public health concern. Further work is needed to confirm this finding
and to understand its mechanism.

Mercury in the thimerosal molecule is in the form of ethyl mercury (CH3CH2-Hg+), for which there is limited toxicologic information. Thus, estimates of health risks from thimerosal in vaccines (7)
were based on the assumption that ethyl mercury is toxicologically
similar to its close chemical relative, methyl mercury (CH3-Hg+),
about which much is known. However, as discussed below, there are
reasons to believe that this assumption is not necessarily correct for
all aspects of the disposition and toxicity of ethyl mercury
compounds, including thimerosal.

History of Human Exposure

Ethyl mercury compounds were first synthesized in the nineteenth century in a chemical laboratory in London (8).
In the late 1880s diethyl mercury was first used in the treatment of
syphilis, a practice soon
abandoned because of the toxic properties of this agent. However, early
in the twentieth century, the fungicidal properties of the
short-chain alkyl mercury compounds led to commercial applications in
agriculture. For example, they are especially effective in a plant
root disease in wheat caused by Telletia triticia. In fact, many different organic mercury compounds were being used to prevent seed-borne diseases of cereal by 1914 (51,52).

Generally
speaking, the ethyl mercury fungicides were used effectively and
safely. However, a number of outbreaks of poisoning occurred in some
developing countries (8). For example, two outbreaks occurred
in rural Iraq in 1956 and 1960 from the misuse use of the fungicide
ethyl mercury toluene sulfonilamide (51). The farmers' families
prepared homemade bread directly from the treated grain instead of
planting it. Hundreds of cases of severe
poisoning occurred, many of which had a fatal outcome. Cases of ethyl
mercury poisoning have occurred in China as recently as the 1970s.
The exposure pathway was the same as in Iraq: The farmers consumed
rice treated with ethyl mercury chloride (53).

Ethyl mercury in the form of thimerosal has found wide application in medicine as a disinfectant. Axton (54)
reported case histories of four children and two adults severely
poisoned by accidental exposure. Five of the six cases died. Rohyans
et al. (55) reported a case of severe poisoning from treatment of an infected ear. Pfab et al. (56)
reported on an attempted suicide from drinking a solution of
thimerosal, resulting in severe poisoning. Treatment of infants with
omphaloceles resulted in high levels of mercury in autopsy tissues (57). Cases of human poisoning have also occurred from infusion of large volumes of plasma
containing thimerosal as a preservative (58,59).

Disposition in the Body

If,
after injection of the vaccine, the thimerosal molecules were to
remain intact for a period sufficient to allow diffusion to the
bloodstream and thence to the kidneys, rapid excretion might take place.
The carboxyl group of thiosalicylic acid might allow thimerosal to be
a substrate for the system responsible for the tubular secretion of
weak acids. Rapid urinary excretion of thimerosal would then be
possible.

This possibility seems unlikely. Thimerosal
contains the ethyl mercury radical attached to the sulfur atom of the
thiol group of salicylic acid. Generally, mercuric ions bind tightly
but reversibly to thiol ligands (60). It is likely, therefore,
that the ethyl mercury cation will dissociate from the thiosalicylic
acid moiety immediately after injection
to bind to the surrounding thiol ligands present in great excess in
tissue proteins.

Thimerosal is used as a thiol titration reagent in numerous experimental studies [e.g., Elferink (61)].
This application would be possible only if rapid dissociation of
ethyl mercury took place in the presence of endogenous thiol groups in
the cells and tissues under study. In vitro, the
thiosalicylate moiety is degraded by oxidation to dithiosalicylic acid
followed by further oxidation to 2-sulfinobenzoic acid (62).

Ulfvarson (63)
demonstrated that the type of anion attached to the alkyl mercury
radical made little difference to the ultimate disposition in the
body. These include such anions as hydroxyl, cyanide, and even the
thiol-containing propane diolmercaptide. These findings suggest that
the mercury radical rapidly dissociates from the anion in the parent
compound to attach to ligands in tissues.

Therefore, it is
assumed that administration of thimerosal results in the immediate
release of the ethyl mercury to the surrounding tissues.
Toxicologically, ethyl mercury in thimerosal is assumed to follow the
same pathways of disposition as ethyl mercury absorbed into the body
from other ethyl mercury compounds.

Patterns of tissue disposition and excretion. Little is known about mercury levels in human tissue after administration of thimerosal. Suzuki et al. (58)
reported levels of total and inorganic mercury in the tissues of a
13-year-old boy who had died 5 days after receiving infusion of
artificial human plasma containing thimerosal as a preservative. The
infusion of plasma had taken place over a period of 6 months, with a
total estimated dose of 284-450 mg Hg. The levels of total mercury
from high to low
were in the following order: liver, kidneys, skin, brain, spleen, and
lowest in plasma. The red cell levels were at least 10-fold higher
than plasma. The distribution pattern is generally similar to that seen
for methyl mercury. These findings are supported by studies in
primates dosed with thimerosal (64).

It is
interesting that hair levels were high. The section proximal to the
scalp had a level of 187 µg Hg/g, whereas the level in blood was
approximately 7 µg Hg/mL, giving a hair-to-blood ratio of 27:1. This is
lower than the commonly assumed ratio for methyl mercury of 250:1, but
possible redistribution of mercury in autopsy blood samples and
uncertainty in the length and exact position of the proximal segment
make the estimates of the hair-to-blood ratio uncertain. However, it
does indicate that ethyl mercury, like methyl mercury, is accumulated
in scalp hair.

Matheson et al. (59) reported on blood
and urine levels in one patient exposed to thimerosal in long-term
injections of gamma globulin. Specifically, they reported on levels of
total and inorganic mercury before and after one injection of gamma
globulin. The data allow a rough calculation of how similar the
observed increase in blood level is to that expected from methyl
mercury. The injected dose was 0.6 mL/kg containing 50.3 µg Hg/mL, to
give a total mercury dose of 30 µg Hg/kg. The disposition parameters
for methyl mercury in adult humans (18) predict that 5% of the
dose--1.5 µg Hg--is deposited in the blood compartment. The volume of
the latter is 70 mL, assuming the blood compartment is 7% of the body
weight. Thus, 1.5 µg Hg would be deposited in 70 mL of blood to give
an increase on concentration of 1500/70 µg Hg/L = 21 µg Hg/L. The
observed increase was 18 µg Hg/L. This calculation
suggests that the disposition of mercury after thimerosal is not very
different from that expected from methyl mercury.

The pattern of urinary excretion also indicates similarities to that with methyl mercury. Matheson et al. (59)
do not quote a specific figure for the change in urinary excretion
after injection of thimerosal, but the graph published in their
article indicates little change. They state that 90% of the total
mercury in urine was in the inorganic form. Adult humans exposed to
methyl mercury excrete little mercury in urine and all in the
inorganic form (10).

Conversion to inorganic mercury. There is, however, one important difference from methyl mercury illustrated in the report from Matheson et al. (59).
Inorganic mercury accounted for about 50% of the total mercury in
blood samples collected from this patient. This is in marked
distinction from methyl mercury, where inorganic mercury accounts for
only about 10% of total mercury in blood (10).

Similar findings were made in the case described by Suzuki et al. (58).
A significant fraction of the total mercury in both gray and white
matter of the brain was in the form of inorganic mercury of the order
of 30-40%. The kidney cortex had the highest percentage. These findings
are confirmed by studies on experimental animals (32). Blood
and tissue levels, including the brain, were higher in animals dosed
with ethyl mercury compared with an equivalent dose of a methyl
mercury compound. The high tissue levels of inorganic mercury seen in
both humans and animals indicate that ethyl mercury breaks down to
inorganic mercury more rapidly than methyl mercury.

Blood levels from thimerosal in vaccines. Stajich et al. (65)
are
the first and only investigators to measure disposition of mercury
before and after administration of vaccines containing thimerosal.
They reported on blood levels of mercury before and 48-72 hr after
administration of a single dose of hepatitis vaccine in the first week
after birth. Seven were preterm infants (average birth weight, 748 g)
and five were term infants (average birth weight, 3,588 g).
Prevaccination blood levels were 0.04-0.5 µg Hg/L. The preterm infant
levels rose to an average value of 7.4 µg Hg/L, whereas the levels in
term infants were 2.2 µg Hg/L.

These levels are similar to
those expected from methyl mercury. The dose was the same for all
infants, 12.5 µg Hg. Five percent, or 0.625 µg Hg, should be deposited
in the blood compartment, which is assumed to be 8% of the infant's
body weight. Thus, for the preterm infants, 0.625 µg Hg would be
deposited in a blood volume of 0.08
748 = 60 mL to give a predicted concentration of 0.625
1,000/60 = 10.4 µg Hg/L. This compares to an observed increase of 6.8
µg Hg/L. The predicted increase for the term infants based on methyl
mercury is 0.625 1,000/287 = 2.2. The observed increase was identical, 2.2 µg Hg/L.

These
estimates also suggest that the disposition of mercury after a dose
of thimerosal is similar to that expected from methyl mercury.
However, these estimates can be regarded as approximate at best.
Individual values for each infant were not reported. The blood levels in
samples collected between 48 and 72 hr may not have been the true
maximum
levels after distribution of the injected dose.

A preliminary report by Pichichero et al. (66) indicated blood levels of mercury in infants lower than what would be expected from methyl mercury. These infants, 6
months of age, had received some vaccines containing thimerosal. Most
blood samples were collected 1 week or more after the last
vaccination. The highest recorded level was 4.1 µg Hg/L, and many were
below the detection limit of about 0.5 µg Hg/L. When the authors
performed calculations similar to those described above, they found
that the methyl mercury dispositional parameters predicted
significantly higher levels than those observed.

The main difference in design of the Pichichero et al. (66) study compared with that of the Stajich et al.
(65) study is that in the former, samples were collected much
later after the last dose of thimerosal. Both studies could be
consistent if the half-time for ethyl mercury in blood is shorter than
that for methyl mercury. The time after collection of 48-72 hr is too
short for a measurable decline in blood levels in the Stajich et al.
study. The urine levels in the Pichichero et al. study were low, which
is consistent with other observations on thimerosal discussed
previously. Significant amounts of mercury were found in fecal samples
that might account for the lower blood levels.

In
conclusion, both animal and human studies indicated that the pattern
of tissue disposition of ethyl mercury was qualitatively similar to
that of methyl mercury, with brain levels of the intact mercury being
slightly higher for methyl than for ethyl. The conversion in body
tissues to inorganic mercury appears to be
substantially faster from ethyl than from methyl. We know little
about the kinetics of elimination of mercury from the body when dosed
with ethyl mercury compounds. The feces is the main pathway of
elimination. The residence time in the body is probably shorter for
ethyl, but quantitative data are lacking.

Adverse Effects

Ball et al. (7)
have reviewed the animal literature on ethyl mercury toxicity.
Toxicity tests conducted before marketing thimerosal in 1931 in
several animal species involved high (45 mg Hg/kg) acute doses with only a short follow-up period (67).
The studies have little relevance to today's concern over risks from
low doses from vaccines. Chronic carcinogenicity studies were
conducted on rats with twice weekly doses ranging from 30 to
1,000 µg Hg/kg. Weight loss was observed at the highest-dose group.
Unfortunately, no brain histopathology was reported, making these
studies difficult to extrapolate to current human exposure.

Magos et al. (32)
compared the target organ toxicity of ethyl and methyl mercury in
rats. Five daily doses of 8 mg Hg/kg were given by gavage for 5
consecutive days. Brain and kidney histopathology was examined 3 and
10 days after the last dose. In general, kidney damage was more severe
after ethyl mercury and brain damage more severe after methyl mercury.
However, when the dose of ethyl mercury was increased by only 20%, the
brain damage was similar or slightly more severe than that seen from
the lower dose of methyl mercury.

Magos (68) has
reviewed the published cases of human poisoning resulting from
exposure to thimerosal. Severe cases of poisoning can result in the
same neurologic signs and symptoms associated with methyl mercury
poisoning, for example, constriction of the visual fields. Ethyl
mercury poisoning was characterized by a latent period of several weeks
between first exposure and onset of the first symptom of poisoning, as
has been observed for methyl mercury. In distinction from methyl
mercury, signs of renal damage are found in severe cases.

A detailed review of case histories on exposure to ethyl mercury including thimerosal allowed Magos (68)
to construct a table comparing blood levels at the time of onset of
symptoms. To estimate such blood levels from samples collected at a
later date, he assumed a half-time in blood of 50 days. Severe
intoxication was associated with blood levels in excess of 2,000 µg
Hg/L, with milder intoxication at 1,000 µg Hg/L. Five cases with blood
levels of 140-650 µg Hg/L had no reported adverse effects. Only 18
cases were involved, with ages ranging from infants to 79 years.
Because of the small number of individuals, no statistical evaluation
is possible in terms of dose-response relationships. However, the data
suggest that ethyl mercury is somewhat less potent in producing
neurologic signs and symptoms than methyl mercury, where the threshold
for neurologic effects has been estimated at about 200 µg Hg/L (18).

Allergic reactions. Allergic
response, usually by skin application, is well known to occur from
organomercurial compounds, including thimerosal (69). Santucci et al. (70)
have demonstrated that contact allergy to thimerosal is due to the
ethyl mercury radical and that it is indistinguishable in its allergic
action from methyl mercury. Goncalo et al. (71) also noted that allergy to thimerosal was mainly related to the mercurial component, but some allergic
reactions may be due to the thiosalicylic acid component.

Allergy
to thimerosal and related mercury compounds is a rare event. There is
evidence that individuals with certain polymorphisms in glutathione
transferase genes may be susceptible to allergic reactions to
thimerosal (72). Glutathione is necessary for the biliary excretion of methyl and inorganic mercury (20), and intracellular glutathione is protective against the toxicity of methyl mercury (29).

Other effects. Only one case of acrodynia has been reported from exposure to thimerosal (59).
This occurred in a 20-year-old man receiving regular gamma globulin
infusions containing thimerosal as a preservative. The total dose was
estimated as 40-50 mg Hg.

Acrodynia is now a rare disease. It was well known to pediatricians when children were exposed to mercury
compounds in teething powders, vermifuge preparations, and diaper disinfectants (73).
The children characteristically have pink hands and feet (hence the
alternative name "pink's disease"). They are photophobic and suffer
from joint pains. A typical picture is that of a child with head
buried in a pillow and continually crying. The distraught parents
invariably take the child for medical attention. Thus, it is unlikely
that cases of acrodynia would escape attention. It is interesting that
not a single case of acrodynia has been reported from exposure to
vaccines despite the propensity of thimerosal to produce this syndrome
when given in sufficient amounts.

An important
characteristic of the disease is that only 1 child in 500 exposed
children develops this disease. The reason for this susceptibility is
not known but presumably has a genetic basis. It has made the
identification of mercury as the
causal agent very difficult (74).

Although it is
unlikely that thimerosal in vaccines has ever caused acrodynia, the
clinical history of this disease gave rise to the idea that a genetic
susceptibility to mercury may underlie other rare childhood diseases.
For example, Bernard et al. (75) claim that autism is a "novel form of mercury poisoning" that occurs in rare infants who are genetically susceptible.

In
attempts to estimate health risks from thimerosal in vaccines, a key
gap in our knowledge on the human toxicology of mercury has become
apparent. Little is known about the tissue disposition and toxicity of
mercury in human infants, or in animals for that matter.

Current
health risks from thimerosal in vaccines depend on the assumption
that ethyl mercury is equally toxic to the nervous system as methyl
mercury.

For example, Ball et al. (7) quote the U.S.
EPA reference dose as a guide for safe intake of ethyl mercury.
However, this reference dose is based on data from prenatal exposures
to methyl mercury. In the case of vaccines, we are dealing with
postnatal exposures. The prenatal stage is believed to be the window
of highest susceptibility to methyl mercury (18). Also,
evidence reviewed above suggests that the systemic toxicity of
thimerosal is less than that of methyl mercury compounds.

Dental amalgam. Dental
amalgam was introduced more than 150 years ago as a tooth filling
restoration. Today it is still the most popular
restorative despite the introduction of new types of fillings. It is
an amalgam of several metals, but mercury is the principal component,
usually accounting for about 50% by weight. Other metals include silver
and copper. Periodically throughout the history of dental amalgam,
concern has been expressed about health risks because of the high
content of mercury. These recurrent concerns have sometimes been
referred to as the "amalgam wars," reflecting the arguments between
the proponents and opponents of its use. Today we are in the third
amalgam war, which started in the early 1970s and continues today
unabated.

This present war was started by reports that
amalgams released mercury vapor that could be inhaled. Concentrations
of mercury vapor in the air in the oral cavity were shown to exceed
occupational health standards. This finding provoked further
investigations and a series of reviews of potential
health risks from amalgam [e.g., World Health Organization (76)].
It was soon realized that comparison with occupational health
standards gave misleadingly high estimates of health risks. The
concentration of mercury vapor in the oral cavity could indeed reach
occupational health danger levels, but the quantity of vapor was small
because the volume of the cavity was small. Eventually more meaningful
data have been obtained indicating that the retained vapor is much
less than that inhaled under conditions of occupational exposures,
except for an interesting exception to be discussed below.

Levels
of mercury vapor in the ambient atmosphere are so low that intake
from this source is negligible. Thus, with the exception of certain
occupational exposures, dental amalgam is the main source of human
exposure to mercury vapor. As discussed further below, other forms of
mercury released from amalgam do
not appear to be important. Thus, a consideration of health risks
from amalgam depends on our knowledge of the toxicology of inhaled
mercury vapor and the quantities released and inhaled from amalgam
restorations.

Mercury vapor. An important source book on this topic is Leonard Goldwater's Mercury: A History of Quicksilver (77). Ramazinni's Diseases of Workers (78),
one of the first books on occupational disease, contains fascinating
historical details of occupational exposures to this metal, as does
Donald Hunter's masterpiece Diseases of Occupations, last printed some 30 years ago (8). The most important recent source books are by the World Health Organization (76), ATSDR (3), and the U.S. EPA (1,2).

Mercury
vapor is a monatomic gas that evaporates from liquid metallic mercury
or is produced by
chemical or physical processes from chemical compounds of mercury.
The principal ore is cinnabar, a brilliant crimson crystalline form of
mercuric sulfide. The largest and oldest mine is located in Almaden,
Spain, which has production records dating back many centuries. These
records show spurts in production as new uses of mercury were
discovered. Perhaps its oldest application was in the form of cinnabar
first used by the Chinese to make red ink for official documents many
centuries before the modern era. Thus, mercury has the dubious
distinction as the founder of bureaucracy.

In its liquid
metallic form, mercury has found innumerable applications. Spread as a
thin film over a sheet of glass, mercury makes an excellent
reflecting surface. An island in the vicinity of Venice, Italy, is
famous for its mirror makers dating back to the middle ages. Ramazinni (78) describes the "mirror makers of
Venice" in these terms:

At Venice on
the Island called Murano where huge mirrors are made, you may see
these workers gazing with reluctance and scowling at the reflection
of their own sufferings in their mirrors and cursing the trade they
have adopted.

Besides mirror making and gilding,
it was also used in the extraction of gold and silver. Enormous
quantities were shipped for extraction of gold and silver from
Almaden, as well as from mines in Peru during the Spanish occupation
of Central and South America. Today, mercury continues to be used in
the largest gold rush of the twentieth century in the Amazon basin (13).
Exposure to mercury vapor and contamination of local fish also
appears to be occurring at other gold mining operations around the world
(79-81).

Liquid mercury
has found important applications in scientific instruments and
measuring devices. It has found its way into many homes in
thermostats, barometers, and thermometers. It has been contained in
household gas regulators. Recent attempts by power companies to replace
such meters has led to spills in the homes because of the careless
nature by which the meters were removed. As many as 200,000 homes in
the Chicago, Illinois, area may have been contaminated in this way (82).

Perhaps
the earliest medical application may have been in ancient Egypt,
where mercury compounds in ointments were used to treat skin
infections. The skin sores from syphilis may have prompted the early
application of mercury to combat this disease as it swept across Europe
soon after the return of Christopher Columbus. Treatment included not
only the application of mercury compounds but also the exposure of the
person's skin surfaces
to mercury vapor. Paracelsus was one of the first advocates for the
mercury treatment, which included skin exposure to the vapor. He soon
realized, however, that a little too much mercury might kill the
patient, hence his famous dictum "Dose makes the poison." So it may be
argued that mercury played a key role in establishing the basic
guiding principle in modern toxicology and risk assessment.

Thus,
the signs and symptoms of poisoning from inhalation of mercury vapor,
at least in its severe form, have been known for centuries if not
millennia. Severe damage to the brain, kidneys, and lungs may result,
depending on the length and intensity of exposure. As discussed below,
today's concerns are with subtle changes in brain and kidney function
associated with occupational exposure and possibly with amalgam under
certain circumstances. Speculations have been put forward that
inhalation of mercury vapor from
amalgam may be a causative factor in chronic degenerative diseases of
the brain such as Alzheimer's disease.

Disposition in the Body

Mercury vapor.
Several recent reviews have discussed in detail the uptake,
distribution, excretion, metabolism, and kinetics of inhaled mercury
vapor (1,3,76). A brief summary is presented here with an update from recent reports.

About
80% of inhaled mercury vapor is retained in the body. However,
approximately 7-14% is exhaled within a week after exposure. The
half-time of the process is about 2 days. The dissolved vapor
accumulates in red blood cells and is carried to all tissues in the
body. It crosses the blood-brain and placental barriers. The half-time
of distribution to the plasma compartment is approximately 5 hr (83).
The amount of time to reach a peak value is 9 hr, with a range
of 7-24 hr in nine adult subjects. The amount of mercury in plasma at
the time of the peak concentration was 4% of the inhaled dose (95%
confidence limit, 3-5%).

Approximately 7% is deposited in the
cranial region after a single exposure to nontoxic levels of the
vapor. The kidney is the main depository.

Once the vapor
has entered the cell, it is subject to oxidation to divalent inorganic
mercury. The oxidation step is catalyzed specifically by the enzyme
catalase, with endogenously produced hydrogen peroxide as the other
substrate. The process is inhibited by ethanol. As a result, workers
imbibing a moderate amount of an alcoholic drink retain less of the
inhaled vapor. The finding that the half-time for exhalation from the
lung is about 2 days suggests that the half-time for the oxidation in
body tissues is about the same.

Studies with radioactive
tracers indicate that the rate of overall excretion of mercury from
the body can be described by a single half-time of about 58 days,
corresponding to an excretion rate of slightly more than 1% of the
body burden per day. Most tissues have the same or shorter half-times.

The decline in plasma levels of mercury consists of at
least two components: a short half-time of less than 1 day and a longer
one of about 10 days. Blood levels therefore reflect recent exposure.

Excretion
takes place via both urine and feces. Urinary mercury originates
mainly from mercury in kidney tissue. Urine is the commonly used
biologic marker, as it reflects the cumulative dose to one of the main
target organs, the kidney. The relationship between urinary excretion
and levels in the other target tissue, the nervous system, is not well
established. As discussed below, urinary mercury levels have been
found to show a rough correlation with signs and symptoms of damage
to the nervous system.

Dental amalgam. Several
studies over the past 30 years or so have demonstrated that amalgam
filling releases mercury vapor into the oral cavity. Mouth breathing
carries the vapor to the lung, where it is absorbed and distributed to
tissues, as discussed above. Mercury levels in autopsy tissue
samples, including the brain, have been shown to correlate with the
total number of surfaces of amalgam restorations. The estimate for the
rate of release in people with amalgam restoration is 2-17 µg Hg/day (18).
The most recent estimate based on applying pharmacokinetic parameters
to steady-state plasma levels in people with amalgam suggests an
average intake between 5 and 9 µg Hg/day (83). Kingman et al. (84),
in a study correlating urinary excretion of mercury with amalgam
surfaces,
estimated that 10 amalgam surfaces would raise urinary levels by 1 µg
Hg/L. As discussed below, these are far below toxic levels. However,
excessive chewing, such as occurs when smokers try to stop smoking by
using nicotine-containing chewing gum, may lead to urine levels in
excess of 20 µg Hg/g creatinine, thereby approaching occupational
health safe limits (85).

Increased amounts of mercury are excreted in feces in individuals with amalgam fillings. Engqvist et al. (86)
found that only 25% of the total mercury in fecal samples was in the
form of amalgam particles in samples taken from six adults with a
moderate load of amalgam fillings. About 80% of an oral dose of
amalgam particles or mercuric mercury attached to sulfhydryl groups
was excreted in the feces. Interestingly, 60% of an oral dose of vapor
dissolved in water was retained. Previously it had been assumed that
intake of vapor
was due solely to inhalation.

Adverse Effects

Mercury vapor. ACUTE
TOXICITY. Cases continue to occur of severe poisoning and even
fatalities from acute exposure to high levels of mercury vapor [e.g.,
see Solis et al. (87)]. Severe lung damage can lead to death
from hypoxia. The poisoning appears to occur in three phases. The
initial phase is characterized by flulike symptoms lasting 1-3 days.
The intermediate phase is dominated by signs and symptoms of severe
pulmonary toxicity. The victim in the final phase will experience
gingivostomatitis, tremor, and erethism (memory loss, emotional
lability, depression, insomnia, and shyness).

The signs and
symptoms of the final phase are identical to those seen in workers
chronically exposed to mercury levels. Generally speaking, such cases
are rarely seen, at least in developed
countries, where industrial hygiene measures are strictly enforced.

THE
NERVOUS SYSTEM. Today, health concerns are directed toward the risk
from lower levels of exposures. In general, air concentrations above
50 µg Hg/m3 in the workplace, corresponding to steady-state
urinary excretion rates of 60 µg Hg/g creatinine, are associated with
fine tremors in the extremities that frequently are not noticed by the
worker (76). Slowed nerve conduction velocity is another preclinical effect found at these lower levels.

Studies on dentists have suggested adverse effects at air concentrations lower that 50 µg Hg/m3 [for review, see Langworth et al. (88)]. Average air concentrations as low as 14 µg Hg/m3
were associated with decreased performance on psychomotor tests.
Changes in mood and behavior have also been noted, such as emotional
lability, somatosensory irritation, and alterations in mood scores.
As noted by Langworth et al. (88), such effects may be due to
mercury exposure. An alternative explanation for the observed
correlations is that "dentists with special personality traits are
less careful in the handling of mercury spills etc. and thus are more
exposed to mercury vapor." If indeed these effects result from
exposure to mercury, one should bear in mind that the average levels
reported in these studies could be substantially less than peak values
that may occur during installation of the amalgam fillings.

Follow-up
studies of workers exposed to high levels of mercury vapor and no
longer exposed during 10 or more years before being examined have
revealed that adverse effects may persist on the nervous system.
Mathiesen et al. (89) examined 70 previously exposed workers (time from last exposure, 1 to 35 years,
average 12.7 years). The average yearly exposure was 8-584 µg Hg/m3.
Peak exposures during any specific year could have been much higher
than these average levels. Decreased performance on a number of
neuropsychologic tests was found, compared with a control group of 52
workers. Despite these high exposure levels, no residual effects were
observed on general intellectual ability or ability to reason logically.

Workers exposed to high levels of vapor at some time during
1953-1966 in a nuclear weapons facility have been the subject of
follow-up studies (90,91). Columns of liquid mercury were used
in the separation of lithium isotopes. The exposure was expressed as
"cumulative average quarterly urine mercury measurements" in units of
micrograms of mercury per liter, from which information one cannot
determine the actual urinary excretion rate (90). However, according to comments in
the text of this paper (90), mercury workers had urine levels in excess of 600 µg Hg/L. In the more recent study (91),
104 of the surviving workers were compared with an unexposed group of
201. Residual adverse effects were found primarily on the peripheral
nervous system. Such long-term adverse effects, as quoted from the
authors, "were not observed for a measure of dementia or other
measures of cognitive function."

As discussed in the
following section on amalgam, a suggestion has been made that inhaled
vapor originating from amalgam filings is a cause or predisposing
factor to Alzheimer's disease (92). The fact that both these studies (90,91)
were unable to detect any signs or symptoms remotely related to this
disease years after heavy exposure to mercury vapor argues strongly
against this suggestion. Many of these workers were exposed for many
years to intakes of vapor
more than 100-fold higher than that experienced from amalgam
fillings.

No new information on adverse effects from prenatal exposure has emerged since previous reviews. However, one study (93) reported that female squirrel monkeys exposed during their pregnancy to air concentations of 500-1,000 µg Hg/m3
had blood levels ranging from 25 to 180 µg Hg/L. No difference was
observed between exposed and nonexposed offspring in various schedules
of reinforcement in terms of lever pressing and other behavioral
measures. The exposed offspring, however, appeared to vary more in the
test performance. Given the high levels of prenatal exposure and the
minimal effects found in the offspring, these data would suggest that
the prenatal period may not be especially sensitive to the effects of
vapor inhaled by the mother. This is consistent with what is known of
the disposition of inhaled vapor in the
maternal-fetal unit. Although vapor passes across the placenta, much
less accumulates in the fetal brain than in that of the mother. The
fetal liver appears capable of oxidizing the vapor in its first pass
through this organ. The product of oxidation, divalent inorganic
mercury, passes the blood-brain barrier far more slowly than the vapor.

KIDNEYS.
Distinct from the action of inorganic mercuric compounds, exposure to
mercury vapor does not produce severe kidney damage. However,
low-level chronic exposures at air concentrations above 50 g Hg/m3 do have adverse effects on the kidney (94).
Decreased selectivity of the glomerular filter is evidenced by
increased excretion of albumin. Tubular reabsorptive function is
slightly diminished, leading to increased excretion of
low-molecular-weight proteins such as retinol-binding protein. Damage to
the brush border of the tubular cells is
indicated by increased urinary excretion of brush border antigens.
Interstitial effects of mercury result in loss of prostaglandins into
the urine. These biochemical markers detect effects of mercury well
before kidney function is significantly compromised.

Taylor et al. (94)
reviewed results from a wide variety of urinary markers. The results
suggest that mercury, lead, and cadmium may produce different patterns
of changes in these markers. The most sensitive tests for the action
of mercury are the tubular and interstitial markers.

MECHANISMS
OF TOXICITY. The mechanism of action of inhaled mercury vapor on
brain function is not known. It is assumed that the vapor is first
oxidized to inorganic divalent mercury that functions as the proximate
toxic agent. The latter can attach to thiol groups present in most
proteins. Thus, almost any enzyme or structural protein is a
potential target.

As discussed previously, it appears that
the intact mercurial and not its metabolic product, inorganic mercury,
is the proximate toxic agent in the neurotoxic action of methyl
mercury. Conversely, we assume divalent inorganic mercury is the
proximate toxic agent after exposure to mercury vapor. The underlying
reason for this apparent conflict is not known. Most likely it is
because of the differences in transport and distribution within the
brain. Methyl mercury is transported as a water-soluble complex that
is metabolized slowly to inorganic mercury only in phagocytic cells not
in neuronal cells. Mercury vapor diffuses to all parts of the brain as
a lipid-soluble monatomic gas that is rapidly oxidized to inorganic
mercury by the catalase-hydrogen peroxide pathway present in all
cells.

Pendergrass et al. (92) have presented evidence
that inhaled vapor
may damage the microtubular system in brain cells in a manner
somewhat similar to that seen for methyl mercury. They reported that
inhaled vapor can inhibit the binding of guanosine triphosphate (GTP)
to a ß subunit of the tubulin dimer. The microtubules of neuronal and
other cells are formed from the polymerization of tubulin protein
subunits in a treadmilling process such that as one end of the
microtubules is formed, the other end is being depolymerized (95).
GTP binding is essential for the polymerization step. Thus, if the
formation step is inhibited, the microtubule will disappear as the
depolymerization continues. Microtubules are key cytoskeletal
structures involved in axonal transport, cell division, and cell
migration. It will be interesting to see if exposure to mercury vapor
leads to disappearance of the microtubules, as has been demonstrated
for methyl mercury.

Consistent with action
on microtubular structures, Leong et al. (96) observed that mercuric ions added in vitro
to cultured neurons inhibited outgrowth and disruptedmembrane
structure. Tests with antibodies for tubulin and actin indicated that
the microtubular structure had disintegrated.

The
inhibitory action of methyl mercury on the assembly of microtubules is
well documented. If further investigation shows that the microtubule
assembly is a common biochemical target for both forms of mercury,
then we face the problem of explaining why the pathology and clinical
signs and symptoms differ so much.

Understanding the
mechanisms of cellular defenses is just as important as understanding
the mechanisms of damage. Thiol-containing molecules probably play a
role in defense as well as being targets for toxicity. Glutathione
complexes with inorganic mercury in liver cells are secreted in bile
and ultimately in the feces. Intracellular levels of glutathione
probably divert mercury from sensitive sites. The thiol-rich family of
proteins known generically as metallothioneins also plays a
protective role. For example, metallothionein has been shown to protect
against kidney damage from inorganic mercury (97). More
recently, it was shown that lung damage was more severe in
metallothionein-null mice than in normal mice after exposure to
mercury vapor (98).

Amalgam. Contact hypersensitivity to mercury is a well-established adverse effect of amalgam fillings [e.g., see Camisa et al. (99)].
According to these authors, a complete remission may be expected
about 3 months after the last amalgam filling is removed.

The
existence of other adverse health effects due to amalgam is presently
unknown but is becoming an area of intensive speculation
and controversy. This is partly because of the limited amount of
research on the safety of amalgam fillings and partly because of the
increased visibility of mercury as a health risk and stringent
regulatory actions concerning this metal.

Ahlqwist et al. (100)
reported on the latest findings of a long-standing study of a cohort
of 1,462 Swedish women established in 1968-1969. Follow-up studies
were conducted in 1974 and 1975, 1980 and 1981, and in 1992 and 1993.
Serum mercury levels correlated with the number of amalgam fillings.
Different clusters of symptoms were recorded as well as the incidence
of diabetes, myocardial infarction, stroke, and cancer. No association
could be found between serum mercury levels and disease in this
population of middle-aged and older women.

The finding that dental amalgam does not affect mental health is from two well-conducted
epidemiologic studies--one on twins in Sweden (101) and the other on older women, the so-called Nun Study in the United States (102). The Swedish study involved approximately 587 subjects from an on-going Swedish Adoption/Twin Study of Aging (103).
The twin study allowed control for genetic predisposition to the
toxic effects of mercury when evaluating the role of amalgam fillings.
No negative effects on physical or mental health were found. The mean
age of the study group was 66 years.

The study on 129
Catholic nuns aged 75-102 years took advantage of a population with
homogeneous adult life styles and environment. No effect of amalgam
status (determined by the number and surface area of the occlusal
surfaces) could be found on eight different tests of cognitive
function.

Cederbrant et al. (104) attempted to address
the possibility that a
susceptible immune system might explain why some individuals with
amalgam fillings claimed to have psychologic, sensory, or neurologic
symptoms from exposure to mercury. They used an in vitro
lymphocyte proliferation assay to test for immune sensitivity to
inorganic mercury on 23 amalgam patients, 30 healthy blood donors with
amalgam, 10 healthy subjects without amalgam, and 9 patients with
oral lichen planus (OLP) adjacent to the amalgam. In addition to the
lymphocyte proliferation assay, a wide range of immune parameters was
measured. None of these end points revealed any significant difference
between amalgam patients and controls despite the fact the in vitro assay was sensitive to the positive control group (OLP).

Because
the inhaled mercury vapor is toxic to the central nervous system,
researchers are now speculating that vapor from amalgams may be a
cause of or an exacerbating
factor in some well-known degenerative diseases such as amyotrophic
lateral sclerosis, Alzheimer's disease (AD), multiple sclerosis, and
Parkinson's disease. Speculation has been most intense concerning AD
after a report that mercury levels were higher in autopsy brains of AD
patients than in brains of members of a control group (105).

However,
subsequent reports have presented an equivocal picture concerning
correlations between tissue levels of mercury and AD. Fung et al. (106)
found no difference between blood mercury levels or mercury to
selenium ratios in AD patients and controls. All subjects resided in a
nursing home, thus ensuring that environmental and dietary exposures
were similar. Conversely, Hock et al. (107) found that blood
levels in AD patients were higher than in controls. In early-onset AD
patients, blood levels were 3 times higher than in controls. Blood
mercury
correlated with concentrations of amyloid ß peptide on the
cerebrospinal fluid in a subset of these patients. Interestingly, the
increases in blood mercury levels were unrelated to the status of
dental amalgam. The reason for the difference in the outcome of the
two studies is not clear, although the study by Fung et al. (106) may have better control over exposure to mercury.

Subsequent studies on brain and related tissues have also discounted a connection between mercury levels and AD. Fung et al. (108)
found mercury levels were the same in various anatomical regions of
the brain in AD patients and matched controls. Cornett et al. (109)
found elevated brain levels in most regions of the brain that were
measured, but no statistical difference could be established from
corresponding mercury levels in control subjects. Mercury levels in
pituitary glands of AD patients were found to
be similar to those of controls (110). In a study of 56 AD patients and 21 controls, Saxe et al. (111)
found no significant association of AD with number, surface area, or
history of having dental amalgam restorations. Mercury levels in the
brain were the same in AD and control patients.

Overall
studies relating tissue levels of mercury to AD have not produced a
convincing picture of any kind of correlation with this disease. Even
if one were established, the "chicken and the egg" issue would arise:
Is mercury the cause of AD or does AD tissue accumulate mercury more
than normal tissue?

Nevertheless, biochemical studies of in vitro
preparations of nerve cells and of AD tissue continue to raise a
question at least on a mechanistic basis that the levels of mercury
may in some way be connected with AD. The brain pathology of AD is
characterized by plaques of
amyloid protein and neurofibrillary tangles (112). The latter consist of altered microtubules and microtubule-associated proteins, especially tau (113)
and are needed for assembly of microtubules from the tubulin
monomers. Phosphorylation of this protein blocks its ability to
promote microtubule assembly. Mercury can interfere with the complex
process of the treadmilling of microtubules.

The study by Leong et al. (96)
on the effect of mercury on neurite growth also noted the appearance
of structures resembling neurofibrillary tangles. The study by
Pendergrass et al. (92), noted that mercury can block the
binding of GTP to tubulin, thus interfering with microtubule assembly.
Other studies have indicated that mercury can cause
hyperphosphorylation of the tau protein (114).

Oxidative stress has been invoked as a cause of AD (115,116).
Mercury is well known to cause biochemical changes in cells is consistent with oxidative stress (114). Indeed, it has been argued that the same enzyme system that protects against oxygen attack also protects against mercury (15).

Such
biochemical observations offer tantalizing possibilities that mercury
can be involved in a mechanism of AD. The process of microtubular
treadmilling is controlled largely by thiol-containing proteins.
Perhaps mercury is simply acting as a thiol reagent and any other
thiol-reactive chemical would produce the same effects. For example, the
lipid peroxidation product 4-hydroxynonenal inhibits neurite
outgrowth, disrupts neuronal microtubules, and modifies cellular
tubulin. Is this also acting by oxidizing thiol groups? As yet we do
not have a complete plausible biochemical mechanism for the genesis of
AD, nor do we know how mercury interferes with this process
in vitro or whether or not mercury acts in vivo.

The
three modern faces of mercury--methyl mercury in fish, mercury vapor
from amalgam tooth fillings, and ethyl mercury in vaccines--represent
our most recent encounter with this ancient metal. Despite thousands
of years of history of human exposure and intense research activity in
our lifetime, many of its toxic actions remain unexplained. This review
reveals key gaps in our knowledge, gaps that highlight important research needs.

The
main features of the disposition of methyl mercury in the body are
well known. Nevertheless, some key gaps remain both in
pharmacokinetics and in the mechanisms of transport and metabolism.

Fecal
excretion is the main pathway of excretion in adults. Animal data
indicate that this process does not start until the end of the
suckling period. However, we have as yet no confirmation in human
infants. Thus, we are unable to estimate the cumulative body burden
from methyl mercury known to be secreted in breast milk. This gap in
our knowledge is especially critical for risk estimates from
thimerosal in vaccines.

Demethylation of methyl mercury by
microflora in the gut is a key, probably rate-determining, process in
the removal of methyl mercury from the body. The microbes involved
have not been identified nor have the biochemical mechanisms of
cleavage of the carbon-mercury bond. The demethylation process in the
gut might well constitute an important site for interaction between
diet and methyl mercury accumulation in the body. The fiber content of
the diet has already been shown to
affect the excretion rate of mercury (117). The diet change at
the time of weaning may also affect the activity and composition of
the microflora. Further studies in this area might shed light on why
there is such a broad range of biologic half-times reported for adults
exposed to methyl mercury.

Molecular mechanisms of transport
of mercury across cell membranes have been identified, indicating
that specific thiol complexes of methyl mercury can enter cells via
the large neutral amino acid carrier and exit on carriers for
glutathione. However, no studies have reported to date on how methyl
mercury gains entry into the hair follicle and then concentrates over a
hundredfold compared to its concentration in whole blood. This is an
important research priority, as head hair is the most widely used
biologic indicator for this form of mercury. If the same entry
mechanism operates for hair follicular
cells as has been shown for endothelial cells of the blood-brain
barrier, then mercury in hair would represent the species of mercury
in blood that enters the brain. This would explain why levels in hair
have been shown to parallel levels in brain.

The long latent
period between the end of exposure and the sudden appearance of
symptoms and signs of neurologic damage is both a fascinating and an
insidious property of the action of methyl mercury on the mature
central nervous system. A slow release of inorganic mercury might
explain this property if the inorganic form were the proximate toxic
species. However, animal experiments indicate that this role is played
by the intact organomercurial moiety. Because the length of the
latent period appears to be independent of the dose, it is also
intriguing and argues against the accumulation of a toxic metabolite.
It we could determine the mechanisms underlying
the latent period, we would learn much more about the toxic action of
the "element of mystery."

Two studies have indicted the
possibility of adverse effects on the cardiovascular system both in
adults and in prenatally exposed children. Such effects appear to be
occurring at methyl mercury levels in the body comparable to those
associated with the lowest levels affecting the central nervous
system. There is an urgent need to confirm these findings in other
populations, preferably where no co-exposure is occurring to other
persistent organic pollutants such as polychlorinated biphenyls.

Generally,
a broad research agenda is needed to develop the toxicology of
thimerosal, given the paucity of our current information. Studies
should be directed to test the assumption that the toxicology of
thimerosal is similar to that of methyl mercury, given the fact the
current estimates of
human health risks, in particular in infants receiving vaccines, are
based on this assumption. The immediate tissue disposition of mercury
following a dose of thimerosal appears to be both qualitatively and
quantitatively similar to that of methyl mercury, as discussed in this
review. However, such limited evidence as now exists suggests that the
rate of conversion to inorganic and, subsequently, the rate of
excretion are more rapid, perhaps substantially so, compared with
methyl mercury. Data on the biologic half-time of the ethyl mercury
radical in body tissues, especially the brain, are essential for
estimates of tissues burdens and health risk from cumulative exposure
from repeated doses of thimerosal in vaccines given to infants. Such
information needs to be gathered both during and after the suckling
period.

Thimerosal also differs from methyl mercury in that
it causes kidney damage at about the
same doses that damage the nervous system. Experimental evidence
indicated that damage to the nervous system is caused by the intact
organomercurial radical, whether methyl or ethyl. However, inorganic
mercury released from ethyl mercury may be the proximate toxic agent
for kidney damage. Indeed, the suspected greater rate of release from
ethyl mercury may explain why kidney damage, if any, occurs only at
the later stages of intoxication from methyl mercury. Thus,
comparative tests of methyl and ethyl mercury should include the
renal-cardiovascular system as well as the nervous system in
developing animals.

It is almost 30 years ago that mercury
vapor was shown to be emitted from dental amalgam fillings. This led
to an outpouring of numerous articles attempting to measure the
precise amounts of vapor released and of factors affecting the release
rate. In general, levels of inorganic mercury in tissue
caused by release of vapor from amalgam are well below those associated
with overt toxic effects or even with subtler neurobehavioral and
renal affects. However, excessive chewing can raise urine levels close
to the lowest safety limits for occupational exposure to mercury
vapor. Interest is now focused on possible indirect effects of vapor
released from amalgam. Despite the fact that several well-conducted
epidemiologic studies have indicated no relationship between dental
amalgam and Alzheimer's disease, speculation continues that the small
amounts of vapor inhaled from amalgam may in some as yet unknown way
exacerbate the progress and severity of this disease. Biochemical
studies reviewed in this article raise intriguing possibilities.

Mechanisms
of cellular resistance toward and defense against these three faces
of mercury have received some research attention. It is suggested that
the focal lesions
produced in the adult brain by methyl mercury are the result not of
selective toxic action but of selective resistance. Those cells having
inadequate defense mechanisms succumb to the initial insult. It is
likely that intracellular glutathione plays a protective role both in
deflecting methyl mercury from sensitive sites in the cell and by
enhancing its exit from the cell. Other thiol compounds, such as the
metallothioneins, may also play a defensive role for both inorganic and
organic forms of mercury (118). More detailed biochemical
information of these defense processes should lead to the
identification of genes controlling cellular resistance and thereby
give some genetic insight into host susceptibility.

As we
gaze at these three modern faces of mercury and reflect upon the
extensive research conducted in our lifetime, we must reluctantly
agree with the title of a BBC documentary broadcast
over 25 years ago that this metal still remains "an element of
mystery."

We read your July 27 guest column entitled "State must adopt strong
mercury controls to protect kids, moms" by Dr. Ed Pont and James
Nelson. We would normally applaud the
acknowledgment of a major environmental health risk in Illinois, but
the authors' prior failings to protect Illinois citizens from another
form of mercury were not as admirable.

While environmental
mercury exposure can be easily sidestepped by dietary restraint,
mercury-laced flu vaccines are much more difficult to avoid in
Illinois. Pont and Nelson represent the Illinois Chapter of the American
Academy of Pediatrics and Illinois Department of Public Health,
respectively, the very organizations that have taken decisive steps to
assure Illinois citizens — particularly children, pregnant women and the
unborn fetus — will continue to be exposed to another toxic form of
mercury contained in most flu vaccines: thimerosal — an ethylmercury
containing compound. The IDPH, at the urging of the ICAAP, filed an
exemption from the Illinois Mercury-free Vaccine Act on Jan. 1, 2006,
an act that would have assured no one in Illinois received
toxic doses of mercury by direct injection. The exemption was filed
under the erroneous pretense that there were not enough mercury-free
flu vaccines available. In fact, public health departments in
California and New Mexico were able to order tens of thousands of flu
vaccines for their citizens, yet Illinois agencies failed to even
inform vaccine providers of the new law signed by Gov. Rod Blagojevich
on Aug. 29, 2005.

Both ethyl- and methylmercury are organic
forms of mercury, the most toxic class of mercury. The methyl form of
mercury found in fish from environmental sources such as coal burning
is a much safer form of mercury. It is already bound to fish proteins
when ingested and therefore is not readily absorbed. In contrast, the
chemical form of mercury found in vaccines is injected directly into
the body, bypassing the defense mechanisms of the gastrointestinal
tract. Even the manufacturers of thimerosal are aware of its
toxic potential:

"Exposure in utero and in children can cause
mild to severe mental retardation and mild to severe motor coordination
impairment," according to Eli Lilly's Manufacturer Safety Data Sheet.

Current scientific studies published in peer-review have strongly
linked the vaccine mercury, thimerosal, to autism and other
developmental disorders.

The only studies which ever attempted
to measure mercury content in children with autism showed statistically
significant greater levels of mercury than in neurotypical children.

The biochemical aberrations in autistic children are identical to those
produced experimentally in the lab when cell cultures are exposed to
thimerosal.

Glutathione, a protein required to remove mercury, is depleted in many autistic children.

The enzyme that is critical in the neurochemical pathway of attention
is also depleted when exposed to mercury, thus providing a
biochemical basis of attention disorders.

There are nine
published epidemiology papers looking at three different U.S.
databases, demonstrating a dose-dependent association between vaccine
mercury and a variety of neurodevelopmental disorders, including autism
and ADHD.

A congressional committee concluded in 2003 that vaccine mercury was the likely cause of the autism epidemic in America.

Thimerosal, injected into mice at equivalent doses of that given
American children before 2003, resulted in behavioral and pathological
brain changes similar to autistic children.

Most importantly,
recent research on juvenile primates proved that the safer mercury
found in vaccines does accumulate in brain tissue at levels over twice
as high compared with methymercury proving its potential for
neurotoxicity.

Pont and Nelson ignore the fact that receiving a single flu shot will result in an exposure to mercury 11
to 16 times greater than what the EPA considers safe and for the fetus, several times above that level.

Is it any coincidence that since the late 1980s when the CDC expanded
the childhood vaccine schedule, autism rates in Illinois have increased
by over 1,500 percent? Currently, 1 in 166 children suffer from a form
of autism, and 1 in 6 from a learning disability.

It is time
to stop blaming coal-burning plants when the blame should squarely lie
on the shoulders of those who are in charge of public health oversight.
A recent international organization assessed infant mortality rates
among industrialized nations. By no surprise the United States ranked
32nd out of 33 participating nations. It is time for parents,
researchers, legislators and physicians to demand that mercury be
removed from all healthcare products. Organizations such as the ICAAP
and IDPH should be held responsible for such an oversight failure. The
fishy story
being spun here has little to do with the fish-eating citizens of
Illinois but by agencies with close ties to pharmaceutical industry
influence. Stop injecting children with mercury and stop this
hypocrisy.